chore: add skeleton of resolver classes

Author: jackwotherspoon

google/cloud/sql/connector/resolver.py

@@ -0,0 +1,37 @@
+# Copyright 2024 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from dns.asyncresolver import Resolver
+
+from google.cloud.sql.connector.instance import _parse_instance_connection_name
+
+
+class DefaultResolver:
+    """DefaultResolver simply validates and parses instance connection name."""
+
+    async def resolve(connection_name: str) -> str:
+        pass
+
+
+class DnsResolver(Resolver):
+    """
+    DnsResolver resolves domain names into instance connection names using
+    TXT records in DNS.
+    """
+
+    pass
+
+
+async def resolve(dns: str) -> str:
+    pass